What Causes Stomach Bloating, Heart Burn And Back Pain With Negative HPT?

Currently, I'm experiencing heart burn, bloating, back pain and difficulty to move fast due to enlargement of my tummy for 5 weeks now. I undertook home pregnancy test, ultra sound and blood test and all were negative.I got period for only 36 hours instead of 5 days last february 9th 2014. All my office mates, relatives and my husband are telling I'm pregnant but my tests are negative.I'm 35 years old, 5'3" tall and weighing 55 kg with regular 27 days menstrual cycle.honestly, with all the symptoms I have, I feel I am pregnant.i could not even wear my trousers and I need to wear dresses because of the increasing size of my tummy.i started drinking folic acid in order to secure my child from birth defects even the test says all negative.i used to wear tight jeans until january this year.now I could not do it anymore.is this a cryptic pregnancy, please help...